Many mobile devices, such as tablet computers, utilize a touchscreen interface instead of a traditional keyboard interface. However, many touchscreen interfaces lack the precision necessary to capture detailed drawings and/or writings (e.g., cursive handwriting). In some cases, a stylus (or other writing utensil) may be used in order to improve the precision of a touchscreen interface. A stylus may be used in combination with either a resistive touchscreen interface or a capacitive touchscreen interface. A resistive touchscreen (i.e., one that detects changes in resistance) may sense where a stylus has made contact with a surface of the touchscreen. A capacitive touchscreen (i.e., one that detects changes in capacitance) may sense where a stylus has made contact with or has come close to a surface of the touchscreen.
Electromagnetic motion tracking technology based on near field electromagnetic propagation has been developed in the context of military applications. For example, electromagnetic coupling has been used to sense the position and/or orientation of a helicopter pilot's helmet during flight. The helmet tracking technology uses three transmitting coils and three receiving coils. The three transmitting coils and the three receiving coils both comprise three coils orthogonal to each other. The three transmitting coils are fixed with respect to a particular coordinate system inside the cockpit of the helicopter. The three receiving coils are attached to the pilot's helmet.
A driving current may be provided to each of the three transmitting coils in a time division manner in order to drive each of the three transmitting coils sequentially. This in turn produces three different magnetic fields, each magnetic field associated with one of the three transmitting coils as it is being driven. As the pilot turns his or her head, an induced voltage across each of the three receiving coils may be sensed in order to determine the strength and direction of the magnetic field generated by each of the three transmitting coils. Per Faraday's law of induction, the induced voltage across a particular receiver coil is proportional to the rate of change of the magnetic flux through the particular receiver coil. By relying on mathematical models (e.g., derived from equations developed from near field or far field electromagnetic theory) of the magnetic fields generated by each of the three transmitting coils, the helmet tracking system may determine the distance and orientation of the three receiving coils relative to the particular coordinate system inside the cockpit of the helicopter.